An Australian man who posted online threats to attack a mosque in Newcastle, NSW, and kill New Zealand Prime Minister Jacinda Ardern has been jailed for 10 months.

Cormac Patrick Rothsey pleaded guilty in Newcastle District Court to charges of posting the extremist threats on social media in September last year.

The posts included statements that he wanted Muslims dead and that Muslims did not belong in Australia.

"My aim is to go to Friday prayer in a packed mosque," Rothsey wrote on Facebook, according to a statement of police facts. "No turning back when called to prayer."

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One post featured an image of Ardern and suggested "she should be put down like an animal".

That post, and others, led counter terrorism police to consider Rothsey an "imminent threat".

Defence lawyer Ben Bickford said while the Facebook posts were highly offensive, Rothsey was simply "lashing out" because he had been assaulted by people he had identified as Muslim men in the past, ABC News reported.

Rothsey was "overwhelmed by thoughts of revenge and hopelessness" but had shown genuine remorse, Bickford said.

He argued there was nothing calculated in the social media posts and had not reached a large audience.

Prosecutor Sam Duggan told the court a term of imprisonment should be imposed, but acknowledged the nine-and-a-half months already served by Rothsey since his arrest.

He said there was clearly a need for mental health treatment as Rothsey was fixated and driven by a dislike of Muslim people.

Judge Tim Gartelmann sentenced Rothsey to 10 months in prison followed by a 14-month recognisance order, under which Rothsey would be required to undergo mental health treatment.

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With time already served, Rothsey will be released from prison on July 4.